1. Field of the Invention
The present invention relates, in general, to a hydraulic system for construction equipment such as power excavators or power loaders and, more particularly, to a hydraulic system used for effectively operating working cylinders or cylinder actuators associated with working units, such as a boom or a bucket, of such construction equipment.
2. Description of the Prior Art
FIG. 1 shows the construction of a typical hydraulic system for working cylinders of construction equipment, for example, a power excavator which is used for excavating and loading bulk materials, such as sand and pebbles.
As shown in the drawing, the hydraulic system has an oil pump P and a working cylinder A. The working cylinder A is, for example, a boom cylinder of the power excavator and is operated by pressurized oil discharged from the pump P. A directional control valve C is mounted to an oil passage extending from the pump P to the boom cylinder A and has a spool, which is movable in either direction in response to a pilot signal PiA, PiB thus controlling the pump discharged oil (pressurized oil) for the boom cylinder A. When the control valve C is positioned at its neutral position, the pump discharged oil is drained into an oil tank T through a bypass line L1. Meanwhile, when the valve C is switched from the neutral position into a first or second position I or II in response to a pilot signal PiA, PiB, the pump discharged oil passes through an oil passage L3, L4 and is introduced into the large or small chamber of the boom cylinder A so that the cylinder A moves the boom of the excavator up or down. When the control valve C is positioned at the first position I in response to the pilot signal PiB, the pump discharged oil is fed into the large chamber of the boom cylinder A through the oil passage L3, with the pressurized oil of the small chamber of the cylinder A being drained into the tank T through the oil passages L4 and L2. Meanwhile, when the control valve C is positioned at the second position II in response to the pilot signal PiA, the pump discharged oil is fed into the small chamber of the boom cylinder A through the oil passage L4, with the pressurized oil of the large chamber of the cylinder A being drained into the tank T through the oil passages L3 and L2. The above-mentioned operation of the system is well known from the construction of the internal passage of the system shown in FIG. 1. The boom cylinder of a power excavator performs a boom-up action when the pump discharged oil is fed into the large chamber of the cylinder and performs a boom-down action when the pump discharged oil is fed into the small chamber of the cylinder.
A relief valve R is mounted to a return passage L5, which extends from both the pump P and the boom cylinder A to the oil tank T. The relief valve R is for maintaining a desirable preset pressure of the system.
However, the above hydraulic system is problematic in that the relief valve R acts as a load in the system when the pressurized oil is drained from the boom cylinder A into the tank T through the return line L2, which prevents the smooth returning of oil into the oil tank T. Therefore, the system causes pressure loss and reduces both operational efficiency and operational speed of the working cylinder or the boom cylinder.
In an effort to overcome the above problem, a hydraulic system for working cylinders shown in FIG. 2 is proposed.
In the hydraulic system of FIG. 2, two return passages L6 and L6 extend from the large and small chambers of a boom cylinder A to an oil tank T, respectively in order to relieve the load caused by the relief valve R. Passage control valve 101, 102 are mounted to each of the two return passages L6 and L7. The system also has a selector valve 103, 104, which is operated in response to a pilot signal PiA, PiB applied to the spool of a directional control valve C and selectively controls each of the two passage control valves 101 and 102. When the pilot signal or pilot pressure PiB is applied onto the spool of the valve C, the pump discharged oil is fed into the large chamber of the boom cylinder A through the oil passage L3 thus causing a boom-up action. The pilot pressure PiB also acts on the back pressure part (closing the passage upon being pressurized) of each of the selector valves 103 and 104 so that the oil passage within the valve 103, 104 is closed by the selector valve 103, 104, with the passage control valve 101, 102 closing the return passage L6, L7 by its valve spring. The hydraulic system in the above state performs the same operation as that described for the system of FIG. 1.
Meanwhile, when the pilot signal or pilot pressure PiA is applied onto the spool of the valve C, the pump discharged oil is fed into the small chamber of the boom cylinder A through the oil passage L4 thus causing a boom-down action. The pilot pressure PiA also acts on the pressure receiving part (opening the passage upon being pressurized) of each of the selector valves 103 and 104 so that the oil passage within the selector valve 103, 104 is opened by the selector valve 103, 104. In this case, the oil, acting on the back pressure part of each valve 101, 102 and closing the return passage L6, L7, is drained into the oil tank T through the selector valve 103, 104 so that the passage control valve 101, 102 opens the return passage L6, L7. When the return passage L6, L7 is opened as described above, the pressurized oil is discharged from the large or small chamber of the boom cylinder A into the oil tank T through not only the valve, but also through the return passage L6, L7, thus effectively preventing pressure loss and improving operational efficiency of the boom cylinder during a boom-down action since the oil from the boom cylinder is returned to the tank T through two passages L4, L5 during a boom-down action. Of course, the preset pressure of the relief valve R of the return passage L5 is lower than that of the relief valve R of FIG. 1 which relieves the load caused by the relief valve R.
As described above, the typical hydraulic system of FIG. 1 causes a pressure loss due to the preset pressure of a relief valve while the pressurized oil is drained from a working cylinder into an oil tank. The above system thus reduces operational efficiency and operational speed of the working cylinder. The system of FIG. 2, which is proposed to overcome the problem experienced in the system of FIG. 1, is problematic in that its operational effect is only expected during a boom-down operation but not during a boom-up operation since only a single passage returning the oil from the cylinder to the tank, that is the passage of the control valve, is provided.